SELECT 
  SUM(product_count) 
FROM 
  cscart_categories 
WHERE 
  FIND_IN_SET(
    2690, 
    REPLACE(id_path, '/', ',')
  )

Query time 0.00370

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "157.71"
    },
    "table": {
      "table_name": "cscart_categories",
      "access_type": "ALL",
      "rows_examined_per_scan": 1513,
      "rows_produced_per_join": 1513,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "6.41",
        "eval_cost": "151.30",
        "prefix_cost": "157.71",
        "data_read_per_join": "4M"
      },
      "used_columns": [
        "id_path",
        "product_count"
      ],
      "attached_condition": "(0 <> find_in_set(2690,replace(`cscart_db`.`cscart_categories`.`id_path`,'/',',')))"
    }
  }
}